This invention aims to reduce the dose applied to patients during computed tomography (CT) imaging and optimize the image quality by a modulation process that continuously fits the applied dose to the momentary attenuation.
The adaptive dose modulation reduces the total dose without significantly increasing noise in the final image and improves the general appearance of the image by reducing the noise streaks in anatomical regions like shoulder and pelvis.
A general computed tomography system has an x-ray source which projects a collimated, fan-shaped beam through the patient towards a bank of radiation detectors. The source and detectors are placed on a gantry that rotates around the patient. The patient table can be shifted inside the gantry, or translated. The angle and position at which the x-ray beam intersects the body can be continuously modified. Each detector produces a signal that is a measure of the body's global transparency from the source down to the detector. The set of detectors' values acquired for a particular source position is referred as a "projection". A "scan" comprises a set of projections made at different gantry or table positions. The CT system acquires many projections during 360.degree. gantry rotation around the patient in order to build a two dimensional image or "slice" through the body. Some of the new CT systems build many slices simultaneously by using multiple rows of detectors. For every projection a "monitor" or reference detector measures the unattenuated beam intensity.
These are two systematic methods for data collection from the patient to produce CT images. The conventional "slice-by-slice" method collects the data for a complete gantry rotation with the patient in a fixed position. Between successive slices the patient is moved to a new position where the next slice can be scanned. This process continue until all planned slices have been scanned.
The "spiral" data acquisition method rotates the x-ray source around the patient while the patient couch is transported continuously through the gantry. The x-ray tube traces a spiral path with respect to the patient until the planned volume has been scanned.
For every CT acquisition mode the image quality is affected by the quantum noise, in order to keep the noise under a certain level, for every projection, the momentary x-ray power level has to be high enough so that the minimum intensity of the radiation leaving the body and reaching the detector is greater than the noise level. Most of the methods that modulate the power profile during the CT scan, used until now need two orthogonal topograms or scout views in order to acquire some information regarding the attenuation profile of the object. From the attenuation information of each topogram line a sinusoidal modulation profile is determined. However, these methods have many drawbacks:
supplementary image noise due to a weak fit of the modulation profile to the real attenuation profile. The two orthogonal projections do not necessarily find the maximum and minimum attenuation of the slice; PA1 not homogeneous noise in the image due to the fact that the modulation profile does not fit to the real attenuation profile; PA1 patient and/or respiratory movements between topogram scan and final scan change the attenuation profile and induce extra errors. PA1 additionally dose used to get the scout projections;